Sorption and desorption of selenium in different soils of the Mediterranean area

The adsorption of selenate selenium and desorption by phosphate in four soils characterized by different physiochemical properties were studied in laboratory. To determine adsorption isotherms of selenate 25 ml of solutions containing 1 to 100 ppm of Se were added to 2.5 g of soil. Desorption isotherms were determined resuspending the samples in phosphate solution. The selenate sorption process was adequately described by the Freundlich equation. In pine forest and woodland soils, characterised by the highest organic matter content and C.E.C. values, the isotherms were classified as L type, since the amount of selenium sorbed appeared to move towards saturation. The organic matter content played the most important part in the adsorption of selenium, while pH appeared to not greatly affect the ability of soil to adsorb Se. The highest CaCO3 content of the pine forest soil could have contributed in increasing the Se adsorption notwhitstanding the high pH value. The cultivated and arable soils showed a reduced sorption capacity and the sorption could be described by an S type? curve. At low concentrations of selenium the affinity of the solid phase was less than that of the liquid phase. Increasing the concentration of selenium in solution, the affinity of the solid phase increased and the sorption was favoured. Selenate desorption by water was negligible, whereas the amount of Se desorbed by phosphate varied among the different soils. Desorption experiments indicated that a noticeable portion of the sorbed selenium was irreversibly retained. This suggests the existence of linkages which allow the release of selenium in the soil solution only after physico-chemical variation such as exchange with phosphate ions.